Wadi Rahjan Monzogranite Reveals REE-Rich Allanite and Bastnäsite in Saudi Arabia

Highlights

• Ferriallanite-(Ce) contains 22.73–30.19 wt% LREE+Y oxides, while hydrothermal bastnäsite reaches up to 70.72 wt% REE oxides at Wadi Rahjan.
• Hydrothermal alteration by fluorine- and CO₂-bearing fluids roughly doubled REE-oxide concentration from allanite precursor to bastnäsite product.
• The study lacks bulk-rock TREO grade, deposit tonnage, metallurgy, and resource definition—key data needed before any economic assessment.
• Next steps require systematic drilling, bulk assays, mineral-resource definition, and beneficiation testing to determine if mineralogy scales to recoverable tonnes.

Adel A. Surour and collaborators Neveen S. Abed, Fayrouz A. Mohamady, and Hesham Mokhtar report in Geochemistry that Saudi Arabia's Wadi Rahjan area hosts a geologically intriguing rare-earth system in which REEs accumulated in magmatic minerals and were later reconcentrated into hydrothermal bastnäsite. The researchers found unusually coarse ferriallanite-(Ce) containing 22.73–30.19 wt% LREE+Y oxides (R₂O₃), while hydrothermal alteration produced bastnäsite containing 49.71–70.72 wt% REE oxides, dominated by lanthanum, cerium, and neodymium. The mineralogy is noteworthy—but this is evidence of REE enrichment, not yet evidence of an economic rare-earth deposit.

REEx Insight: Rich Minerals Do Not Yet Make a Mine

The investor signal is the mineralogy. Bastnäsite is a commercially important REE mineral, while xenotime-(Y) and the study's yttrium-rich columbite broaden the system's mineralogical interest. Most intriguing, hydrothermal alteration concentrated REEs substantially: the authors report bastnäsite with roughly twice the REE-oxide concentration of its allanite precursor.

But REEx draws a bright line between mineral composition and orebody grade. The headline percentages measure individual minerals—not bulk-rock TREO grade, deposit tonnage, or a mineral resource. A bastnäsite grain containing 70.72 wt% REE oxides does not mean Wadi Rahjan hosts 70% TREO ore.

From Magma to Rare-Earth Concentration

Researchers combined geological mapping and Landsat imagery with mineralogical and geochemical analysis, including back-scattered electron imaging and SEM-EDS, to reconstruct the Nu'man monzogranite and its alteration history.

Allanite accounts for more than 90% of the accessory minerals described by the authors. Later fluorine- and CO₂-bearing hydrothermal fluids partially to extensively replaced ferriallanite-(Ce) with bastnäsite. Associated chamosite indicates crystallization at approximately 214–232°C. The authors propose fluorite as the fluorine source and possible descending meteoric water as the source of CO₂.

Promising Geology, Missing Economics

The study does not establish a mineral resource, representative bulk TREO grade, deposit tonnage, metallurgy, recovery rate, mining geometry, or project economics. That is its central investment limitation.

Next should come systematic sampling and drilling, bulk-rock assays, mineral-resource definition, detailed Nd-Pr-Dy-Tb distribution, mineral liberation work, and beneficiation and hydrometallurgical testing.

Saudi Arabia may have another intriguing REE occurrence. The next test is whether compelling mineralogy scales into economically recoverable tonnes.

Citation: Surour AA, Abed NS, Mohamady FA, Mokhtar H. Depletion of radioelements and enrichment of rare-earth elements in accessory minerals of the Wadi Rahjan monzogranite, Saudi Arabia: Magmatic allanite vs. hydrothermal bastnäsite. Geochemistry. 2026;86(4):126467. doi:10.1016/j.chemer.2026.126467.